1. Field of the Invention
This invention relates to a control for the hydraulic system in vehicle lift racks in which the supporting runways are elevated by single action pressure fluid rams and are lowered by the weight of the vehicle on the runways or simply by gravity.
2. Description of the Prior Art
Applicant is aware of hydraulic automobile lift mechanisms having runways adapted to be elevated by mechanical interconnecting devices operated from a common hydraulic power source. The problem with such mechanisms is that they are required to have below-ground excavations for some of the operating devices. Examples of the foregoing prior art are 2,201,147 of May 21, 1940; 2,201,189 of May 21, 1940; 2,208,983 of July 23, 1940; and 2,201,147 of June 10, 1941.
In the field of above ground surface-mounted lifting mechanisms, applicant is aware of such art as 3,289,868 of Dec. 6, 1966; 4,706,458 of Nov. 17, 1987; and 4,724,930 of Feb. 16, 1988. Other examples of mechanisms having multiple lifting rams requiring synchronous operation are 3,494,259 of Feb. 10, 1970; 3,760,688 of Sept. 25, 1973; 4,403,680 of Sept. 13, 1983; and 4,526,346 of July 2, 1985.
The problems with the noted prior art examples are the complex construction devised to handle loads requiring elevation, leakage that can develop after a period of use, and lack of accuracy in obtaining simultaneous movement of the separate lifting runways to protect the load from tipping. That is especially important when the load is an automobile or truck. An additional problem in handling fluid displacement rams is how to maintain the volume of fluid substantially the same in each flow system.